An updated parameterization for infrared emission and absorption by water vapor in the National Center for Atmospheric Research Community Atmosphere Model

• Published in: Journal of Geophysical Research - Atmospheres Vol. 107; no. D22; pp. ACL 17-1 - ACL 17-20
• Main Authors: Collins, William D., Hackney, Jeremy K., Edwards, David P.
• Format: Journal Article
• Language: English
• Published: American Geophysical Union 27-11-2002; Blackwell Publishing Ltd
• Subjects: continuum; GCM; General circulation; longwave cooling; Meteorology and Atmospheric Dynamics; Numerical modeling and data assimilation; Radiative processes; radiative transfer; water vapor
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/2001JD001365

An updated parameterization for the absorption and emission of infrared radiation by water vapor has been developed for the Community Atmosphere Model (CAM) from the National Center for Atmospheric Research (NCAR). The CAM is the latest version of the NCAR Community Climate Model (CCM). This updated treatment preserves the formulation of the radiative transfer equations using the absorptivity/emissivity method. However, the components of the absorptivity and emissivity related to water vapor have been replaced with new terms calculated with the General Line‐by‐line Atmospheric Transmittance and Radiance Model (GENLN2). The mean absolute errors in the surface and top‐of‐atmosphere clear‐sky longwave fluxes for standard atmospheres are reduced to less than 1 W/m2. Mean absolute differences between the cooling rates from the original method and GENLN2 are typically 0.2 K/d. These differences are reduced by at least a factor of 3 using the updated parameterization. The updated parameterization increases the longwave cooling at 300 mbar by 0.4 to 0.7 K/d, and it decreases the cooling near 800 mbar by 0.2 to 0.6 K/d. The increased cooling is caused by line absorption and the foreign continuum in the rotation band, and the decreased cooling is caused by the self‐continuum in the rotation band. These changes in the vertical profile of longwave cooling interact strongly with the parameterization of convection. The effects on the fluxes, diabatic cooling rates, and climate state are illustrated using simulations with the new climate model.